[SI-LIST] Re: Guard Rings ... DC

  • From: steve weir <weirsi@xxxxxxxxxx>
  • To: keithK EPD <epd2001usa@xxxxxxxxxxx>
  • Date: Fri, 19 Nov 2010 07:46:59 -0800

Keith most if not all of your questions have been answered.  Just search 
the archives for guard rings.  Guard rings take advantage of Ohm's Law.  
The leakage current between some signal line and a conductor of any 
other potential is proportional to the voltage difference between the 
nodes, and inversely proportional to the resistance between them.  A 
guard ring of any kind sets the potential difference.  When the guard 
ring is actively driven to closely match the voltage of the sgnal lead 
it guards, then leakage currents can be made very small with ordinary 
resistivity PCB materials.

Steve
keithK EPD wrote:
> Thanks for your responses so far - I have read alot of articles as such ...
>  
> For the most part I see a lot of good practices...
> I'm hoping that Ansoft can bring to light the more analytical approach to 
> many trade offs to attain the desired goal.
>  
> This is what I found out so far:
> 1) clean is essential.
> 2) can't mask vapor deposition paralene coating - guard ring masking is 
> pretty hard to do (I don't know why masking will help....please shed some 
> light on this)
> 3) baking the boards after assembly increases volume resistivity
> 4) I wish i understood why the guard ring works - in my mind its' a common 
> practice, but.....
> 5) using other material other than FR-4 increase the volume resistivity - see 
> Rogers Corp 
> 6) distance between electrodes is better... 
> 7) looking for a cost effective solution - teflon is too much / plus it's a 
> soft material / need something rigid like fr-4
> 8) slots are great , cost of pcb increases
>  
>  
> I think some should write a program for use in battery applications and hi-z 
> amplifiers
>
>
>
> Keith Kowal
> www.product-designs.com
>
>  
>  
>  
>
>
>  
>
>   
>> Date: Fri, 19 Nov 2010 05:35:07 -0800
>> From: weirsi@xxxxxxxxxx
>> To: epd2001usa@xxxxxxxxxxx
>> CC: si-list@xxxxxxxxxxxxx
>> Subject: [SI-LIST] Re: Guard Rings ... DC
>>
>> Keith there are articles on this sort of stuff from the makers of very 
>> input impedance amps: ADI, TI, NS, LTI, etc. The trade-off with 
>> conformal coating is that the coating will give you a lot more leakage 
>> than a clean, dry PCB. If the board can be kept clean and dry then 
>> avoid the conformal coat. There are other tricks that have been 
>> previously mentioned involving packaging the Hi Z amplifier off the PCB 
>> using PTFE stand-offs and such.
>>
>> Steve
>> keithK EPD wrote:
>>     
>>> I am dealing with DC ! so, some of my questions are not for the entire High 
>>> speed audience, so I apologize 
>>>
>>> I have a 4 gig ohm electrode/electrodes(s) ..... my electrode spacing is 
>>> 0.230" and my socket has a 0.175" diameter / 4 layer
>>>
>>> I haven't seen much lit on the subject - 
>>>
>>> PCB material / voids / components / etch width to the 1 resistor into the 
>>> opAmp / given 0.060" / slots req'd 
>>> what does conformal coating do for me....
>>>
>>> comments, pointers to white papers, etc are welcome.
>>>
>>>
>>> Keith Kowal
>>>
>>> www.product-designs.com
>>>
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>> -- 
>> Steve Weir
>> IPBLOX, LLC 
>> 150 N. Center St. #211
>> Reno, NV 89501 
>> www.ipblox.com
>>
>> (775) 299-4236 Business
>> (866) 675-4630 Toll-free
>> (707) 780-1951 Fax
>>
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>>     
>                                         
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-- 
Steve Weir
IPBLOX, LLC 
150 N. Center St. #211
Reno, NV  89501 
www.ipblox.com

(775) 299-4236 Business
(866) 675-4630 Toll-free
(707) 780-1951 Fax


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